1. Field of the Invention
The present invention relates to a vehicle suspension apparatus and, more particularly, to an improvement in suspension for reducing pitching occurring in a vehicle body during traveling.
2. Description of the Related Art
In a conventional suspension apparatus, if a vibration period input from a road surface is close to the natural frequency of suspension when a vehicle travels on a waved road having a long wave cycle, large pitching may occur in the vehicle, or adjustment of the vehicle position becomes difficult. Therefore, a suspension apparatus which can reduce pitching occurring in the vehicle body is proposed. This apparatus comprises suspension units interposed between wheels and a vehicle body and having fluid spring chambers, a fluid supply unit for supplying a fluid to the fluid spring chambers through supply valves, a fluid exhaust unit for exhausting a fluid from the fluid spring chambers through exhaust valves, a vehicle height sensor for detecting a distance between the wheels and the vehicle body, and a control unit for controlling the supply and exhaust valves. With this arrangement, the control unit is designed to open the supply valves for a set or predetermined time in response to an upward displacement of vehicle height and to open the exhaust valves for a predetermined time in response to a downward displacement of vehicle height when a vibration period of vehicle height detected by the vehicle height sensor falls within a predetermined range with respect to a reference vehicle height as the center of the range and its amplitude is larger than a predetermined value, thereby reducing pitching occurring in the vehicle body.
In this apparatus, however, if a threshold value used for performing control for reducing pitching is set to be excessively small, since compressed air is consumed every control operation, the operation frequency of a compressor or pump serving as the fluid supply unit or the number of times of actuation of the supply and exhaust valves is increased. Therefore, durability of each component is inevitably degraded. In contrast to this, if the threshold is set to be excessively large, even intermediate-level pitching cannot be reduced.